Aircraft control is typically achieved through the manipulation of control surfaces located on the wings and tail surfaces of the aircraft. Typical control surfaces include the elevator, rudder, and ailerons. When a control surface is moved, the aerodynamic forces acting on the control surface create a reaction force that is transmitted back through the control linkages to the pilot control yoke, giving the pilot a “feel” to the control of the aircraft. A pilot can immediately feel how much control is being exerted through the control yoke before the effect is manifested in a change in aircraft attitude.
Control feedback is thus a valuable aid in the flying of an aircraft. The aforementioned aerodynamic reaction forces, however, are not constant for all flying conditions such as airspeed and altitude. This gives rise to a residual force or back pressure that must be maintained continuously in steady level flight. In some aircraft, this force may be substantial, potentially creating significant pilot fatigue and pilot distraction. Maintaining an aircraft in a state of trim prevents the aircraft from deviating from the intended flight path, i.e. if the pilot were to release the control momentarily in an out of trim state the aircraft would change heading, altitude or bank, and furthermore, under certain conditions, the aircraft may quickly enter an uncomfortable or potentially dangerous attitude.
To counter the residual control force, various trim mechanisms may be installed to balance the residual force for a given flight condition. A trim tab installed on the control surface is a commonly chosen solution. The trim tab may be adjusted by the pilot to neutralize pressure for any given flight condition. Current trim technology on light aircraft consists of either manual control or electric control of moving trim tabs, bias assemblies, or other similar devices. These systems require input from a pilot to set the trim position. During critical flight maneuvers, trimming the aircraft can be a distraction for the pilot. Furthermore, manual systems can be very slow to operate depending upon their location and design. Electrically operated trim systems allow the pilot to make adjustments from a normal sitting position without having to reach for a manual trim wheel or other control, but can still be very slow during operation to avoid a pilot overshooting the target trim position, requiring the pilot to move the trim in the opposite direction. This can lead to a dangerous condition of the pilot over controlling the trim. Most electric trim systems are therefore designed to move slowly for more precise control and prevent over controlling the trim.
A system and method to automate trim control of a light aircraft during hand flown flight operations therefore would eliminate the need for pilot action, reducing pilot fatigue and distractions during critical flight maneuvers. Furthermore, an automatic trim control system would maintain an aircraft in a constant state of trim, thereby reducing flight path deviations.